A variety of photographic imaging systems are known wherein a hardenable organic component containing ethylenic unsaturation sites is relied for image formation. The organic component undergoes photoinduced addition reactions, typically either polymerization or crosslinking, at the ethylenic unsaturation sites which produce hardening and allow image discrimination to be achieved.
It is common practice in preparing these compositions to employ coinitiators. One of the coinitiators is a photosensitizer. Photosensitizers are relied upon to capture photons of exposing radiation. The remaining coinitiator is referred to as an activator. The activator is not relied upon to respond directly to exposing radiation, but rather adjacent activator and photosensitizer molecules react, following excitation of the latter by photon capture, causing release by the activator of a free radical which in turn induces immobilizing addition reactions at sites of ethylenic unsaturation.
It is generally accepted that photosensitizer coinitiators function by photon absorption to lift an electron from an occupied molecular orbital to a higher energy, unoccupied orbital. The spin of the electron lifted to the higher energy orbital corresponds to that which it exhibited in its original orbital or ground state. Thus, the photosensitizer in its initially formed excited state is in a singlet excited state. The duration of the singlet excited state is limited, typically less than a few nanoseconds. The excited photosensitizer can return from its singlet excited state directly to its original ground state, dissipating the captured photon energy. Alternatively, the singlet excited state photosensitizer in some instances undergoes intersystem crossing through spin inversion to another excited state, referred to as a triplet state, wherein lifetimes are typically in the microsecond to millisecond range. Since photosensitizer coinitiators which exhibit triplet states remain in an excited state for time periods that are orders of magnitude greater than photosensitizer coinitiators which exhibit only singlet excited states, a much longer time period is available for reaction with the paired activator coinitiator.
Specht and Farid U.K. No. 2,083,832A discloses photopolymerization coinitiators including azinium activators and amino-substituted 3-ketocoumarin and naphthothiazole merocyanine photosensitizer coinitiators which exhibit triplet states on excitation. While most of the photosensitizers absorb primarily in the ultraviolet portion of the spectrum, some are yellow dyes with absorption maxima ranging up to 480 nm in wavelength. An essentially cumulative disclosure is provided by Research Disclosure, Vol. 200, December 1980, Item 20036. Research Disclosure is published by Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England. As illustrated by Specht, Martic, and Farid, "A New Class of Triplet Sensitizers", Tetrahedron, Vol. 38, pp. 1203-1211, 1982, these amino-substituted 3-ketocoumarins exhibit intersystem crossing efficiencies ranging well above 10 percent--e.g., from 0.18 to 0.92 or 18 to 92 percent.
Specht and Farid in comparative examples demonstrate the performance of ethyl diethylaminobenzoate as an activator in place of the N-oxyazinium activators disclosed. The results show that the ethyl diethylaminobenzoate exhibits comparatively low speeds with amino-substituted coumarins as compared to oxy-substituted coumarins.
In concurrently filed, commonly assigned patent applications, Ser. Nos. 933,658 and 933,660, tilted NEGATIVE WORKING PHOTORESISTS RESPONSIVE TO SHORTER WAVELENGTHS AND NOVEL COATED ARTICLES and NEGATIVE WORKING PHOTORESISTS RESPONSIVE TO LONGER WAVELENGTHS AND NOVEL COATED ARTICLES, respectively, negative working photoresists are disclosed comprised of a film forming component containing ethylenic unsaturation and capable of selective immobilization as a function of ethylenic addition, and activator and photosensitizer coinitiators for ethylenic addition. The activator is an azinium salt, and the photosensitizer is a dye having a reduction potential which in relation to that of the azinium salt activator is at most 0.1 volt more positive.
In concurrently filed, commonly assigned patent application, Ser. No. 933,712, titled DYE SENSITIZED PHOTOGRAPHIC IMAGING SYSTEM essentially similar azinium salt and photosensitizer cointiators are disclosed in combination with a image dye or its precursor and an organic component containing ethylenic unsaturation sites and capable of being hardened imagewise as a function of addition reactions at the unsaturation sites to immobilize selectively the image dye or its precursor.